1. Field of the Invention
The invention relates to a method of writing and subsequent reading of information in a magneto-optical information layer of a record carrier by means of diode laser radiation, which radiation is focused to a diffraction-limited radiation spot on the information layer, and by means of a movement of said radiation spot and the record carrier relative to each other, information being written by locally changing, at the location of the radiation spot formed by the write beam, the direction of magnetization of areas in the information layer by means of a diode laser write beam and a magnetic field, the information areas thus formed being read by detection of the variation, caused by the information areas, of the state of polarization of a diode laser read beam. The invention also relates to an apparatus for performing the method and to a magneto-optical record carrier suitable for use of said method.
2. Description of the Related Art
Such a method and apparatus are known from, for example the Article "Experiments towards an erasable Compact Disc Digital Audio System" in "Audio Engineering Society", 73 Conv., 1983, pp. 1-14 and the associated nine Figures. The apparatus described in this Article comprises a diode laser for supplying a write beam as well as a read beam. The two beams are focused to a diffraction-limited radiation spot having a half value width of the order of 1 .mu.m. The size of the written information area, in the form of a magnetic domain, is determined by the dimension of this radiation spot. In the known system the information area is round and has a diameter of the order of 1 .mu.m and the information density is of the order of 400,000 bits per mm.sup.2. The information density may be increased to approximately 1,000,000 bits per mm.sup.2.
There is an ever increasing need of information densities which are larger than the ones stated above, so that more information can be recorded in a record carrier of the same dimensions. To this end it must be possible to write and read information areas in a magneto-optical record carrier, which areas are smaller than the ones that have hitherto been conventional. Within the present concept of magneto-optical information recording in which one radiation source and one radiation spot are used for writing and reading, the envisaged object can be realised by reducing this radiation spot.
Since the size of the diffraction-limited radiation spot is proportional to .lambda./NA, in which .lambda. is the wavelength of the radiation used and NA is the numerical aperture of the objective system used, the radiation spot can only be reduced by decreasing the wavelength and/or increasing the numerical aperture. An increase of the numerical aperture involves a decrease of the depth of focus of the radiation beam, so that the requirements imposed on the focusing of the radiation beam will become more stringent. Moreover, an objective system having a larger numerical aperture is more sensitive to aberrations so that more stringent tolerance requirements must be imposed on the write-read apparatus. If it is desirable to continue the use of a diode laser as a radiation source, which is necessary in a mass product which the magneto-optical write-read apparatus is intended to be, it is not a real possibility to decrease the wavelength of the radiation beam, because there are no short wavelength diode lasers which have a sufficiently high power for the writing operation.